QueuingBeamFnDataClient adds polling latency to completing bundle processing

Currently the inboundDataClients are registered with recieve, and they add data to a queue. There is no explicit indication from the clients that they are no longer going to add values to the queue.

Within QueueingBeamFnDataClient.drainAndBlock the queue is therefore polled and if nothing is present all clients are polled to see if they are complete.

This design makes for unfortunate tradeoffs on poll timeout:

• cpu wasted with small timeout
• additional latency in noticing we have completed with larger timeout

With the existing InboundDataClient interface, we could have a separate thread call awaitCompletion on all of the clients and then shutdown the queue (adding a poison pill perhaps)
Or we could modify InboundDataClient interface to allow registering iterest in when the client is done producing elements. The existing clients all seem based upon futures which allow that.